Communication systems, such as the telephone system (e.g., the Public Switched Telephone Network or PSTN) typically employ bridged taps, which are open circuit cable pair segments constructed such that multiple branches of a single pair share a common origin. Bridged taps are provided so that a common pair of wires can serve different subscribers. The common pair of wires passes more than one subscriber location, with each location having an access point for connecting the subscriber's service drop wire to the common pair. Bridged taps are commonly incorporated into telephone distribution networks in order to provide plant flexibility for future additions, or for changes in service demands.
FIGS. 1 and 2 illustrate two different types of bridged tap constructions. Shown in FIG. 1 is a tree-type topology bridged tap construction in which a number of cable pairs (collectively bundle 102) emanate from respective line cards at the central office 104. At point 106, bridged tap 107 connects to cable pair 109. Bridged tap 107 is a cable pair within bundle 108, which includes a number of cable pairs, only one of which is shown in the drawing. Bridged tap 112, which is within bundle 110, is shown as a continuation of cable pair 109. Similar to bundle 108, bundle 110 contains a number of cable pairs, only one of which is shown (112). Connected to cable pair 112 is a drop cable which provides service access for subscriber 114. The topology of FIG. 1 is "tree-type" in that the bridged taps resemble interconnecting branches of a tree. In contrast, the bridged tap construction of FIG. 2 is a bus-type construction in that the distribution path from the central office is a single path, with no branching paths.
Such bridged tap construction results in a cable layout having one or more cables starting at a common origin. Each of these cables may have branches, and the branches may in turn have additional branches. The resulting topology is the tree-type topology having branching cables with no closed loops. Alternatively, where there is no branching, the topology may reduce to a bus-type topology in the case of a single cable with one or more subscribers attached to the shared common line.
A class of digital subscriber line transmission methods (e.g., ADSL, VDSL) use a single pair of wires to provide both narrowband and broadband services to a subscriber using frequency division multiplexing. This is illustrated in FIG. 3. At the network side of the line, narrowband and broadband services are coupled onto the line at the same location (e.g., at the central office or at a remotely located service node). Specifically, narrowband services are coupled via line card 120, while broadband services are coupled via ADSL modem 122. The narrowband and broadband services are coupled onto cable pair 112 within bundle 102 via respective service filters 124 and 126. Alternatively, the narrowband and broadband services may be coupled onto the line at different locations on the network side of the line (e.g., Plain Old Telephone Service POTS coupled onto the line at the central office and ADSL coupled onto the line at a remotely located service node).
At the subscriber side of the line, the services are separated at the end of the drop at the customer premises 128 using respective service filters 130 and 132. The service filters used in combining and separating services may be such as those described in U.S. Pat. Nos. 5,627,501 and 5,528,630, the contents of which are incorporated herein by reference. For example, a filter may be located at the end of the drop wire which couples the telephone service onto the existing in-premises telephone wiring and isolates the broadband service from the telephone wire pairs. Alternatively, the two service filters 130, 132 at the customer premises 128 may be combined into one apparatus, for example, a POTS-type splitter. A metallic wire pair has information carrying capacity (bandwidth) available in the unused frequency spectrum of the channel. In the case of a POTS line, the frequency spectrum from 0-4 kHz may be used for POTS, while the upper portion may be used for ISDN, ADSL or VDSL.
The number of metallic wire pairs in adequate condition may be insufficient to support full market deployment of broadband services using the existing telephone network infrastructure, because certain limitations exist on the number and quality of wire pairs in the distribution cables. The number of wire pairs that pass each premises is usually limited. For example, in the case of residential premises, the number of wire pairs that pass the premises is typically two pairs, with a minimum of one and a maximum of about five pairs. It is estimated that the number of pairs available may be insufficient to meet the demand for services.
Several aspects of the existing telephone network infrastructure limit the information carrying capacity (bandwidth orbit rate) of the individual lines. These factors include: (1) the type of cables and the use of bridged taps; (2) the condition of the plant; and (3) the noise picked up by the network. Also, a reduction in capacity may result from restricted use of particular frequency bands (e.g., amateur radio bands) because of potential radio frequency interference. These factors will reduce either the usable bandwidth available for broadband service (bit rate) or the length of the working line (reach).